Cellular and Molecular

T here are many different types of molecular motor; some ofthem move linearly along polymers, whereas others undergorotary motions (Table 1). Many other nucleotidases, including tubulin, actin, chaperones and some G proteins, should perhaps be considered as molecular motors as well because the generation of mechanical strain is central to their activity. However, the primary focus of this review is on linear cytoskeletal motors. For detailed information on the structure, mechanism, superfamily composition and biological roles of molecular motors, the reader is referred to recent collections of reviews on these subjects (Box 1). Origins of molecular motors Molecular motors are exquisitely complicated and masterful devices. A great deal of insight into the workings of these protein machines comes from X-ray crystallography and other structural studies. At the heart of most molecular motors is the ATPase site, which binds to ATP, hydrolyses the b–g phosphate bond and then releases phosphate and ADP. These enzymatic transitions elici